Valve timing control device for an internal combustion engine

ABSTRACT

A valve timing control device for an internal combustion engine. The device comprises first determination means for determining a first optimal value of the valve overlap period in the current engine operating condition, on the basis of the current engine speed and load, second determination means for determining a second optimal value of the valve overlap period in the current engine operating condition by reducing the first optimal value when the engine has not warmed up, and valve overlap period control means for controlling a valve overlap period using the first optimal value when the current degree of opening of the throttle valve is larger than a predetermined value, even if the engine has not warmed up. A back-flow of exhaust gas does not occur when the degree of the throttle valve is relatively large so that the valve overlap period at this time is made larger and a higher engine torque can be obtained to increase the trapping efficiency and the scavenging efficiency, even if the engine has not warmed up.

This application is a division of application Ser. No. 08/312,586, filedon Sep. 27, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a valve timing control device for aninternal combustion engine, the device comprising a variable valvetiming mechanism for controlling the valve overlap period.

2. Description of the Related Art

When the valve overlap period, during which the intake valve and theexhaust valve are opened simultaneously at the end of an exhaust stroke,is made long, the trapping efficiency and the scavenging efficiencyincrease so that a good performance at high engine speed can beobtained. On the other hand, in the idle condition, the long valveoverlap period causes a back-flow of a large amount of exhaust gas sothat combustion deteriorates. Accordingly, in an idle condition, it isdesirable that the valve overlap period is made short. For this purpose,a variable valve timing mechanism capable of varying the valve overlapperiod has already been suggested.

A known valve timing control device for controlling the valve overlapperiod by means of the variable valve timing mechanism determines anoptimal valve overlap period in accordance with a current engineoperating condition on the basis of current engine load and enginespeed. Moreover, Japanese Unexamined Patent Publication No. 4-194331discloses a valve timing control device which makes the optimal valveoverlap period determined for each engine operating condition shorterwhen the engine has not warmed-up because combustion at this time isunstable in contrast with when the engine is warmed-up.

According to the above valve timing control device, in each engineoperating condition when the engine has not warmed up, the amount ofback-flow exhaust gas is reduced so that combustion can become stable.However, in high engine load operating conditions, when the engine hasnot warm up, the required engine torque cannot be obtained.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a valvetiming control device for an internal combustion engine, capable ofrealizing stable combustion in each engine operating condition and therequired engine torque in high-engine-load operating conditions when theengine has not warmed up.

According to the present invention there is provided a valve timingcontrol device comprising: first determination means for determining afirst optimal value of the valve overlap period in the current engineoperating condition, on the basis of the current engine speed and load;second determination means for determining a second optimal value of thevalve overlap period in the current engine operating condition byreducing the first optimal value when the engine has not warmed up; andvalve overlap period control means for controlling a valve overlapperiod using the first optimal value when the current degree of openingof the throttle valve is larger than a predetermined value, even if theengine has not warmed up.

The present invention will be more fully understood from the descriptionof the preferred embodiments of the invention set forth below, togetherwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic view of a valve timing control device, accordingto the present invention;

FIG. 2 is a first flow chart for controlling the valve overlap periodvia a variable valve timing mechanism by an electronic control unit;

FIG. 3 is a first map for determining an optimal value of the valveoverlap period in accordance with an engine operating condition;

FIG. 4 is a second flow chart for controlling the valve overlap periodvia a variable valve timing mechanism by an electronic control unit;

FIG. 5 is a time chart of the valve overlap period controlled by thesecond flow chart;

FIG. 6 is a third flow chart for controlling the valve overlap periodvia a variable valve timing mechanism by an electronic control unit;

FIG. 7 is a time chart of the valve overlap period controlled by thethird flow chart;

FIG. 8 is a fourth flow chart for controlling the valve overlap periodvia a variable valve timing mechanism by an electronic control unit;

FIG. 9 is a second map for determining a first value and a second valuein accordance with an engine speed;

FIG. 10 is a graph showing the generating rate of the engine torqueagainst the degree of opening of the throttle valve; and

FIG. 11 is a third map for determining a correction value in accordancewith the temperature of the cooling water.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic view of a valve timing control device, accordingto the present invention. In this figure, reference numeral 1 designatesa cylinder head of an engine and reference numeral 2 designates avariable valve timing mechanism. Various types of the variable valvetiming mechanism have already been suggested. Here, one of them issimply explained as follows. Reference numeral 2a designates a cam shaftfor the intake valve and reference numeral 2b designates a pulley whichis rotated by a crank shaft (not shown) via a belt 3. The pulley 2 isfixed to an outer sleeve 2c which surrounds the end of the cam shaft 2a.Outer teeth (not shown) are formed around the end of the cam shaft 2aand inner teeth (not shown) are formed around inside wall of the outersleeve 2c. Moreover, an intermediate gear (not shown) which has outerand inner teeth is arranged between the outer sleeve 2c and the end ofthe cam shaft 2a. The outer teeth of the cam shaft 2a and the innerteeth of the outer sleeve 2c mesh the inner and outer teeth of theintermediate gear, respectively. The inner teeth of the outer sleeve 2cand the outer teeth of the intermediate gear are formed as helical teethso that when the intermediate gear is moved axially by oil pressure andthe like, the cam shaft 2a is relatively rotated against the outersleeve 2c. Thus, the variable valve timing mechanism 2 can vary thevalve overlap period freely by changing the opening time of the intakevalve in accordance with this relative rotation of the cam shaft 2a.

Reference numeral 20 designates an electronic control unit (ECU) forcontrolling the valve overlap period via the variable valve timingmechanism 2. The ECU 20 is constructed as a digital computer andincludes a ROM (read only memory) 22, a RAM (random access memory) 23, aCPU (microprocessor, etc.) 24, an input port 25, and an output port 26.The ROM 22, the RAM 23, the CPU 24, the input port 25, and the outputport 26 are interconnected by a bidirectional bus 21.

An engine speed sensor 11 which produces an output pulse for detectingthe engine speed is connected to the input port 25. In addition, an airflow meter 12 produces an output voltage which is proportional to theamount of intake air fed into the engine cylinder, and this outputvoltage is input into the input port 25 via an AD converter 27a. Athrottle valve sensor 13 produces an output voltage which isproportional to the degree of opening of the throttle valve (not shown),and this output voltage is input into the input port 25 via an ADconverter 27b. A coolant temperature sensor 14 produces an outputvoltage which is proportional to the temperature of the cooling water ofthe engine as the engine temperature, and this output voltage is inputinto the input port 25 via an AD converter 27c. The output port 26 isconnected to the variable valve timing mechanism 2 via a drive circuit28.

The ECU 20 controls the valve overlap period via the variable valvetiming mechanism 2, according to a first flow chart shown in FIG. 2.First, at step 101, the current engine speed (N), the current amount ofintake air fed into the engine cylinder (G), the current degree ofopening of the throttle valve (TA), and the current temperature of thecooling water (THW) are detected by the above four sensors 11, 12, 13,14, respectively. Next, at step 102, a basic optimal value (AB) of thevalve overlap period in the current engine operating condition isdetermined from a first map, shown in FIG. 3, on the basis of thecurrent engine load (G/N) (an amount of intake air fed into the enginecylinder per unit engine speed) and the current engine speed (N). In thefirst map, a basic optimal value (ABnm) for each engine operatingcondition is set, such that the smaller the engine load (G/N) becomes,the smaller the basic optimal value (ABnm) becomes to realize stablecombustion, because the negative pressure in an intake port at a lowengine load is relative high so that if the valve overlap period is madelong, the amount of back-flow exhaust gas becomes large, and the largerthe engine load (G/N) becomes, the larger the basic optimal value (ABnm)becomes to increase the trapping efficiency and the scavengingefficiency to take account of the current engine speed (N).

Next, the routine goes to step 103, it is determined if the currenttemperature (THW) of the cooling water of the engine is equal to or ishigher than a predetermined lower limit value (THW') which representsthe engine warmed-up temperature. When the result is positive, theroutine goes to step 104 and a valve overlap period (A) at this time isset to the basic optimal value (AB) determined at step 102. Next, theroutine is stopped.

On the other hand, when the result at step 103 is negative, the routinegoes to step 105 and an optimal value (AB') when the engine has notwarmed up and is in a cold condition is calculated by subtracting acorrection value (K) from the basic optimal value (AB). Next, at step106, it is determined if the current degree of opening of the throttlevalve (TA) is larger than a predetermined value (TAW). When the resultis negative, i.e., when the degree of opening of the throttle valve (TA)is not very large, the routine goes to step 107 and a valve overlapperiod (A) at this time is set to the optimal value (AB') for the coldcondition. Next, the routine is stopped. Thus, when the degree ofopening of the throttle valve (TA) is not very large in the coldcondition, the valve overlap period (A) at this time is made smallerthan the basic optimal value (AB) so that the amount of back-flowexhaust gas is further reduced and in each engine operating condition,deterioration of combustion in the cold condition is prevented.

On the other hand, when the result at step 106 is positive, i.e., whenthe degree of opening of the throttle valve (TA) is very large, theroutine goes to step 104. Thus, when the degree of opening of thethrottle valve (TA) is very large, pressure in the intake port is aboutatmospheric pressure so that back-flow of exhaust gas does not occur inspite of the large valve overlap period. Whereby, if the valve overlapperiod at this time is made smaller than the basic optimal value (AB),the combustion cannot be improved and the trapping efficiency and thescavenging efficiency are reduced. Accordingly, at step 104, the valveoverlap period (A) is made the basic optimal value (AB) so that thetrapping efficiency and the scavenging efficiency increase and a higherengine torque can be obtained.

The ECU 20 may control the valve overlap period according to a secondflow chart shown in FIG. 4, instead of the first flow chart. Thedifference between the first and second flow charts is explained asfollows.

In the second flow chart, when the engine has not warmed up, it isdetermined at step 206 if the current degree of opening of the throttlevalve (TA) is larger than the predetermined value (TAW). When the resultis positive, the routine goes to step 208 and the valve overlap period(A) is gradually varied from the optimal value (AB') in the coldcondition calculated at step 205, to the basic optimal value (AB)determined at step 202, as shown in FIG. 5.

According to the second flow chart, when the degree (TA) of opening ofthe throttle valve exceeds the predetermined value (TAW) in the coldcondition, the valve overlap period is at first set to the optimal value(AB') in the cold condition so that the difference between the valveoverlap periods at this time and at the last time is relatively smalland the large engine torque variation is not generated. Thereafter, thevalve overlap period is gradually increased to the basic optimal value(AB) so that the engine torque can be increased without the large torquevariation.

The ECU 20 may control the valve overlap period according to a thirdflow chart shown in FIG. 6, instead of the second flow chart. Thedifference between the second and third flow charts is explained asfollows.

In the third flow chart, when the engine has not warmed up, it isdetermined at step 306 if the current degree (TA) of opening of thethrottle valve is larger than the predetermined value (TAW). Thereafter,when the result is positive, the routine goes to step 308 and the valveoverlap period (A) is calculated using the following expression (1).

    A=(AB-AB')*(TA-TAW)/(100%-TAW)+AB'                         (1)

According to the expression (1), as shown in FIG. 7, when the degree(TA) of opening of the throttle valve reaches the predetermined value(TAW), the valve overlap period (A) is made the optimal value (AB') inthe cold condition. Moreover, when the degree (TA) of opening of thethrottle valve reaches the full open value (100%), the valve overlapperiod (A) is made the basic optimal value (AB), and when the degree(TA) of opening of the throttle valve is a value between thepredetermined value (TAW) and the full open value (100%), the valveoverlap period (A) is made a value in accordance with the current degree(TA) of opening of the throttle valve.

According to the third flow chart, when the degree (TA) of opening ofthe throttle valve exceeds the predetermined value (TAW) in the coldcondition, the large torque variation is not generated as in the secondflow chart. Moreover, the engine torque can be increased as the driverdesires because the valve overlap period is increased in accordance withthe degree of opening of the throttle valve.

The ECU 20 may control the valve overlap period according to a fourthflow chart shown in FIG. 8, instead of the third flow chart. Thedifference between the third and fourth flow charts is explained asfollows.

In the fourth flow chart, when the engine has not warmed up, at step 406it is determined if the current degree (TA) of opening of the throttlevalve is larger than a first value (TAW1). Here, the first value (TAW1)is not a constant as the predetermined value (TAW) in the previous flowcharts, and is a variable which vary in accordance with a current enginespeed (N). The first value (TAW1) is set in a second map shown in FIG.9, such that the lower the engine speed becomes, the smaller the firstvalue (TAW1) becomes. When the result at step 406 is positive, theroutine goes to step 408 and a calculation value (A') of the valveoverlap period is calculated using the following expression (2).

    A'=(AB-AB')*(TA-TAW1)/(TAW2-TAW1)+AB'                      (2)

Next, the routine goes to step 409 and it is determined if thecalculation value (A') is larger than the basic optimal value (AB). Onlywhen the result is positive, does the routine go to step 410 and thecalculation value (A') is set to the basic optimal value (AB). Next, theroutine goes to step 411 and the valve overlap period (A) is set to thecalculation value (A').

The expression (2) uses the first value (TAW1) instead of thepredetermined value (TAW) and a second value (TAW2) instead of (100%),in contrast to the expression (1). Here, the second value (TAW2) is avariable which vary in accordance with a current engine speed (N) assame as the first value (TAW1), and is always larger than the firstvalue (TAW1) as shown in the second map (FIG. 9).

As seen from the expression (2), when the degree (TA) of opening of thethrottle valve reaches to the first value (TAW1), the calculation value(A') of the valve overlap period becomes the optimal value (AB') in thecold condition. Moreover, when the degree (TA) of opening of thethrottle valve reaches to the second value (TAW2) prior to the full openvalue (100%), the calculation value (A') becomes the basic optimal value(AB), and when the degree (TA) of opening of the throttle valve is avalue between the first value (TAW1) and the second value (TAW2), thecalculation value (A') becomes a value in accordance with the currentdegree (TA) of opening of the throttle valve. In the expression (2),when the degree (TA) of opening of the throttle valve is larger than thesecond value (TAW2), the calculation value (A') becomes larger than thebasic optimal value (AB) but the calculation value (A') is set to thebasic optimal value (AB) at step 410.

According to the fourth flow chart, in the cold condition, when theengine speed is relatively low, the valve overlap period (A) is broughtfrom the optimal value (AB') in the cold condition close to the basicoptimal value (AB) even if the degree (TA) of opening of the throttlevalve is smaller than in the third flow chart. Since a required amountof intake air at a low engine speed is smaller than at a high enginespeed, even if the valve overlap period at low engine speed is madelarger, the amount of back-flow exhaust gas is slightly increased, andthe trapping efficiency and the scavenging efficiency are sufficientlyincreased, and thus the engine torque at this time can be increased.

Moreover, in the cold condition, when the engine speed is relative low,the valve overlap period (A) reaches the basic optimal value (AB) evenif the degree (TA) of opening of the throttle valve is smaller than(100%). Accordingly, as seen from a graph shown in FIG. 10, in eachengine speed operating condition, the relationship between the degree ofopening of the throttle valve (TA) and the generating rate of the enginetorque (in each engine operating condition, the ratio of the actualgenerating torque to the maximum generating torque does not to takeaccount of a deterioration in combustion) becomes linear. In the thirdflow chart, when the valve overlap period is brought from the optimalvalue (AB') close to the basic optimal value (AB) at a low engine speed,the increasing rate of the engine torque against the degree of openingof the throttle valve is relatively small, as shown by the dot line ofthe graph. However, according to the fourth flow chart, the enginetorque in each engine speed operating condition is sufficientlyincreased, and thus, the vehicle moves as the driver desires.

In the previous embodiments, the correction value (K) is a constant tosimplify the control for the valve overlap period. However, it may be avariable as shown in a third map (FIG. 11) so that the valve overlapperiod can be controlled more favorably to take account of the enginetemperature.

Finally, the variable valve timing mechanism 2 can vary the valveoverlap period smoothly. However, it is clear that the present inventioncan be realized by a variable valve timing mechanism which varies thevalve overlap period in stages.

Although the invention has been described with reference to specificembodiments thereof, it should be apparent that numerous modificationscan be made thereto by those skilled in the art, without departing fromthe basic concept and scope of the invention.

I claim:
 1. A valve timing control device for an internal combustionengine comprising:first determination means for determining a firstoptimal value of a valve overlap period in a current engine operatingcondition, on the basis of a current engine speed and load; seconddetermination means for determining a second optimal value of the valveoverlap period in the current engine operating condition by reducingsaid first optimal value when the engine has not warmed up; and valveoverlap period control means for controlling a valve overlap periodusing said first optimal value when a current degree of opening of athrottle valve is larger than a predetermined value, even if the enginehas not warmed up.
 2. A valve timing control device according to claim1, wherein said second determination means determines said secondoptimal value by subtracting a constant correction value from said firstoptimal value.
 3. A valve timing control device according to claim 1,wherein said second determination means determines said second optimalvalue by subtracting a variable correction value from said first optimalvalue, said variable correction value being set such that the higher theengine temperature becomes, the smaller said variable correction valuebecomes.
 4. A valve timing control device for an internal combustionengine comprising:first determination means for determining a firstoptimal value of a valve overlap period in a current engine operatingcondition, on the basis of a current engine speed and load; seconddetermination means for determining a second optimal value of the valveoverlap period in the current engine operating condition by reducingsaid first optimal value when the engine has not warmed up; and valveoverlap period control means for controlling a valve overlap period suchthat when a current degree of opening of a throttle valve is larger thana first value which is set at each engine speed, said valve overlapperiod is brought from said second optimal value close to said firstoptimal value in accordance with a ratio of said current degree ofopening of the throttle valve to a second value which is set at eachengine speed and is larger than said first value, even if the engine hasnot warmed up.
 5. A valve timing control device according to claim 4,wherein said valve overlap period control means controls a valve overlapperiod according to the following expression when the current degree ofopening of the throttle valve is larger than said first value, even ifthe engine has not warmed up:

    A=(AB-AB')*(TA-TAW1)/(TAW2-TAW1)+AB'

where, (A) represents a valve overlap period, (AB) represents said firstoptimal value, (AB') represents said second optimal value, (TA)represents said degree of opening of the throttle valve, (TAW1)represents said first value, and (TAW2) represents said second value. 6.A valve timing control device according to claim 4, wherein the higherthe engine speed becomes, the larger said first and second valuesbecome.